DURBAN, South Africa (Reuters) - A boom in world tourism is posing a huge threat to some
of the planet's most sensitive ecosystems, according to a study released on Friday.

The study, by Conservation International (CI) and the United Nations
Environment Program (UNEP), said tourism rose by more than 100 percent between 1990 and
2000 in the world's "biodiversity hotspots," which include the tropical Andes and the
Guinean forests of West Africa.

CI has identified 25 such areas, which contain 44 percent
of all identified endemic plant species and 35 percent of all known endemic species of birds,
mammals, reptiles and amphibians.

The hotspots cover only 1.4 percent of the planet's land
area and all been significantly altered by human activities.

"In some places the growth (in tourism) has been staggering," CI and UNEP said in a statement
released at the fifth World Parks Congress in the South African port city of Durban.

"Over the past decade, tourism has increased by more than 200 percent in both Laos and Cambodia,
nearly 500 percent in South Africa, (and) over 300 percent in the countries of Brazil, Nicaragua
and El Salvador," it said.

Costas Christ of CI, one of the report's authors, told Reuters tourist development in ecologically
sensitive areas often severely damages its main attraction -- the environment.

"Many of these developments are in arid countries and so the limited water supply comes under
pressure...and if development ultimately kills off the environment then tourists have no incentive
to come," he said.

The report highlighted the Mexican resort of Cancun, where world trade talks are being held,
as an example of unsustainable tourism which is impacting negatively on the environment.

"Prior to its development as a tourist resort in the 1970s, only 12 families lived on the barrier
island of Cancun," it said. Now, the resort has 2.6 million visitors per year, the local mangrove
and inland forests have been cut down, and in the settlement that has grown nearby, 75 percent
of the sewage of the population is untreated.

Tourism is often said to benefit the environment by creating jobs and other opportunities for
poor rural communities who might otherwise exploit local natural resources for survival.